Conventionally, as filters for applications such as masks and air conditioning, electrically charged (electret) nonwoven fabrics have been typically used, which are nonwoven fabrics formed of polypropylene fiber electrically charged by a method such as the corona discharge method or the hydro-charging method. These electrically charged nonwoven fabrics including polypropylene fiber, however, are poorly heat resistant. Therefore, these electrically charged nonwoven fabrics are unsuitable for various exhaust gas filters and filtration materials (heat-resistant filters) used in heat-resistance-requiring applications such as collection and removal of high-temperature dust and the like discharged from diesel engines and the like.
It is known that a nonwoven fabric including heat-resistant fiber is conventionally used as a heat-resistant filter for such heat-resistance-requiring applications. For example, Japanese Patent Laying-Open No. 2009-119327 (PTD 1) discloses a lightweight heat-resistant filter including a nonwoven fabric that contains heat-resistant short fiber such as aramid fiber, polyether ether ketone fiber, polyimide fiber, PPS fiber, polytetrafluoroethylene fiber, polyester fiber, nylon 66 fiber, or phenol fiber. According to Japanese Patent Laying-Open No. 2011-183236 (PTD 2), a heat-resistant filter including a heat-resistant fiber such as polyphenylene sulfide fiber, meta-aramid fiber, para-aramid fiber, polyamide-imide fiber, or polyimide fiber is also known.
Japanese Patent Laying-Open No. 2010-90512 (PTD 3) discloses a layered structure composed of two or more layers for use as a filtration material for a filter. The layered structure consists of a super-ultrafine-fiber layer that has a fibrous structure including super ultrafine fiber of wholly aromatic polyamide or the like and a nonwoven-fabric layer including thermoplastic fiber of polyphenylene sulfide, polyether ether ketone, polyether ketone, a thermoplastic polyimide, or the like. These layers are bonded together by thermocompression bonding at respective areas thereof having a linear, corrugated, or zigzag shape, and are thereby formed into a single component. According to the description of PTD 3, this layered structure has excellent collection efficiency owing to the super-ultrafine-fiber layer and also has very low pressure loss, so that it is suitable for use as a heat-resistant filter for removing hazardous substances in gases discharged from iron factories, thermal power stations, garbage incinerators, and coal boilers, for example.
Heat-resistant filters including glass fiber, and the like have also been widely known. However, the heat-resistant filters of this type have poor handleability because they have great specific gravity (weight) and they cause irritation to skin due to scattered fibers. In addition, the heat-resistant filters of this type cause waste treatment problems.
Therefore, there has been a demand for development of a novel filtration material having improved performances although various heat-resistant filters have been conventionally known as described above.